The present invention relates to a particle impactor used for classifying particles according to size for analysis of the particles carried in a gas flow. Sharp particle cutoff is obtained, with compactly arranged impactor/collection cups and flow channels for conserving space, and providing accurate information about the particles.
Various types of impactors have been utilized in the prior art, including devices that use cascading elements for obtaining a classification of particles. Generally a cascade impactor has a plurality of collection stages arranged in series, with each stage having a nozzle or orifice plate with nozzle openings smaller in size than those of the previous stage, and also having an impaction surface for the collection of the particles that are too large to be carried farther in the fluid stream. At the smaller nozzle openings, the velocity of the fluid carrier is higher, and the particles have a higher velocity moving through the nozzle. The higher the flow velocity through the nozzle, the smaller the particles that are collected on the impaction plate. In other words, particles larger than the cut size of an impactor will impinge upon the impaction surface and the rest of the particles will pass with the fluid or airstream to the next stage. The particles that are collected at each stage can be analyzed by weight, or by quantitative chemical analysis. When the particles are to be chemically analyzed, it is desirable to collect the particles in a container or cup so the particles easily can be transported to a lab for analysis.
Another problem that arises with impactors is loss of particles due to the collection of some particles on surfaces other than the particle impactor surface, the collection on surfaces other than the impactor surface results in losses and these are called interstage particle loss. Minimizing such interstage particle loss is a desirable feature of the present invention
The present invention relates to a compact, high productivity cascade impactor that is easily used, manually or with a robot system and which provides for a broad flow range with quite precise particle cutoff sizes at the various stages. The physical construction makes the impactors of the present invention easily automated, and the usual final filters can be eliminated from the system and a microporous plate filter provided to avoid errors that may arise by contaminants on conventional filters.
The interstage passageways and the nozzles are designed so that they have low particle losses. The impactors thus are acceptable to regulators, such as the Food and Drug Administration and the British and European equivalents.
The impactor preferably has cups that are supported on a tray or frame. The cup tray and all the cups can be removed as a separate unit for quantitative recovery of material from the cups. The cup shape is chosen to reduce the space occupied by the impactor while not compromising the aerodynamic performance of the impactor.
A preferred form of the invention conserves space by utilizing teardrop shaped interstage passageways and collection cups. Other forms include stacked impactors, in order to provide for unique and compact units. The impactors are preferably constructed of inert materials, and are physically robust, and since they are metal and can be grounded they are unaffected by static.
The present invention is made with one set of fixed nozzles to achieve the desired range of particle size cut points, from about 0.5 xcexcm to 10 xcexcm at seven different cut points. At any flow rate, five or more of these are within the range desired for assessing the safety and efficacy of the drug formulation. The impactor is designed to accept a wide range of flows, for example between 30 liters per minute and 100 liters per minute, which is the typical flow range for testing inhalables as described in the United States Pharmacopoeial (USP) or corresponding British, European or Japanese Pharmacopoeials.